1. Field of the Invention
The present invention relates to the field of shared multipoint-to-multipoint communication networks. The present invention will be described hereafter in relation with an example of application to networks using the electric supply conductors (for example, the mains) as a transmission medium. High-frequency carriers which are modulated to transmit data between two or several devices equipped with modems and connected to the mains are generally used. Such networks may be used, for example, to connect a microcomputer to its peripherals (printer, scanner, etc.). They may also distribute, inside of a home, an office, or the like, multimedia data coming from a connection to an external access, for example, a satellite antenna, an optical fiber cable, a modem cable, an XDSL modem, etc.
2. Discussion of the Related Art
Since various devices may simultaneously need to transmit information over the shared network, an access control mechanism of the transmission means (MAC) is necessary to avoid collisions which would result in information losses. Further, since different multimedia services or transmission types are likely to use the same electric supply conductors and since these different transmissions most often have distinct constraints in terms of delay, bit error rate, etc., an access priority management mechanism more generally designated as a quality-of-service control (QOS) is generally used.
FIG. 1 very schematically shows an exemplary architecture of a transmission network using electric supply conductors L as a transmission medium. The electric supply network connects different taps P together, possibly via an electric board provided with circuit breakers or the like (not shown). Taps P have been symbolized in FIG. 1 as being taps with three conductors (phase, neutral, and ground). However, these also may be taps only having two conductors (phase and neutral). From the point of view of the transmission network, each tap is considered as a node. Six nodes N1, N2, N3, N4, N5, and N6 are shown in FIG. 1.
Among the various electric devices connected to the network, devices 1 of a first type equipped with a modem Mk (k ranging between 1 and 4 in the illustrated example) are respectively connected to nodes Nk to communicate over the network. In the example of FIG. 1, a modem-free device 2 is connected on one of the taps. Device 2 is only supplied by the electric network.
Most often, each device 1 of the first type connected to any tap from the point of view of the electric supply (node from the point of view of the network) should be able to send and receive data. The network accordingly is a so-called point-to-multipoint or multipoint-to-multipoint network.
The data transmission protocols over shared networks can be grouped in three large categories. A first category concerns time-division multiple accesses (TDMA or TDD), which assign different time slots to each transmission. A second category concerns frequency-division multiple accesses (FDMA or FDD), which assign one or several frequencies to each transmission. A third category concerns code-division multiple accesses (CDMA) and multicarrier code-division multiple accesses (MC-CDMA) which assign, for each transmission, different codes, also called spreading sequences or matrixes.
All these transmission systems are generally used in multipoint-to-point systems such as, for example, GSM mobile telephony communication systems. They however all have disadvantages in the point-to-multipoint or multipoint-to-multipoint communication systems to which the present invention applies.
For time-division transmissions, significant dead times must be provided between each data sequence sent by each device. These dead times must be inserted to avoid collisions between packets transmitted by different nodes, while taking account of all possible reflections and multiple paths between the transmitter and the receiver. This disadvantage can significantly reduce the network capacity due to the decrease in general transmission rate.
For frequency-division multiple accesses, analog filters are required to separate the frequencies or frequency groups used for the transmit and receive sections of the modems. This makes the system less flexible and less adaptable in frequency since the analog filters which are formed cannot be modified according to the dynamic capacity needs or any other reason requiring modification of the assigned frequencies. This is a significant disadvantage, especially in the case of a network using the power conductors as a transmission support. Indeed, the transfer function of such a network considerably varies along time (for example, upon plugging of an electric device, be it or not equipped with a modem) and from one node to another.
For simple code-division transmissions (CDMA), that is, single-carrier transmissions, flow rate limitations on the order of a few hundreds of kilobits per second (at most, a few megabits per second) are observed in practice due to the complexity of managing the multiple users which causes a lot of interference, and to the level differences of the received signal according to the paths to be followed by the different signals. The implementation of an efficient code-division system requires significant means, which can quickly reach a prohibitive cost. Further, the spectral density of a code-division transmission extends over the entire usable bandwidth, which makes this type of transmission incompatible with electromagnetic compatibility requirements, which require being able to forbid transmissions in certain specific frequency bands.
The present invention mode specifically relates to a multicarrier code-division transmission (MC-CDMA) which, with current techniques, remains unadapted to point-to-multipoint or multipoint-to-multipoint transmissions.
FIG. 2 shows a synoptic diagram illustrating a simplified case of operation of an MC-CDMA coding and modulation means 4. This simplified case assumes the transmission of a single datum d(k)(n), where n represents the time rank of the datum in the data flow to be transmitted, spread over N carriers by the modem of a k-th user of a network. It is also assumed, as an example and to simplify, that the transmission channel is perfect (its frequency response is ideal). Datum d(k)(n) is provided to a number N of multipliers 6. Each multiplier 6 is provided to multiply datum d(k)(n) with an element c(k)(m) of a sequence, called a spreading sequence, m ranging between 0 and N−1. The output of each multiplier 6 is provided to a corresponding modulator 8 to be modulated on a carrier at frequency (fc+m.F/Tb). fc is the first frequency of a group of frequencies or carriers of the multicarrier modulation. F/Tb is the interval between two consecutive carriers, F being an integer chosen according to the sampling frequency and to the frequency band used by signal OFDM (orthogonal frequency-division multiplexing) and Tb being the duration of datum d(k)(n), except for a guard interval. All carriers are added in an adder 10 to form an OFDM symbol, or time signal, S(k)(n, t). Symbol S(k)(n, t) is a digital signal formed of a sequence of samples. It should be noted that, in this specific example, modulators 8 and adder 10 may altogether be implemented by an inverse Fourier transform (IFFT). The spreading sequences assigned to the network nodes are chosen to be orthogonal to one another, that is, so that the sum of the products of the elements of same rank of any two spreading sequences of different nodes is zero. This amounts to respecting the following formulas 1 and 2 where 1 and p are integers ranging between 1 and the maximum number TN of nodes:
                                          ∑                          m              =              0                                      N              -              1                                ⁢                                                    c                                  (                  1                  )                                            ⁡                              (                m                )                                      ·                                          c                                  (                  p                  )                                            ⁡                              (                m                )                                                    =                  1          ⁢                                          ⁢                      (after  normalization)                                              (        1        )            
if 1 is equal to p; and
                                                        ∑                              m                =                0                                            N                -                1                                      ⁢                                                            c                                      (                    1                    )                                                  ⁡                                  (                  m                  )                                            ·                                                c                                      (                    p                    )                                                  ⁡                                  (                  m                  )                                                              =          0                ,                            (        2        )            
if 1 is different from p.
The network receives the sum of all symbols S(i)(n, t) transmitted by each node i.
To receive the preceding datum d(k)(n) from a given node j of the network, the sum of the symbols present on the network is provided to a demodulation and decoding means (not shown). First, the sum of the symbols is demodulated for each carrier m. It should be noted that the demodulation can be implemented by a fast Fourier transform (FFT). The result of the demodulation for carrier m is substantially equal to sum:
                              ∑                      i            =            0                    TN                ⁢                                            d                              (                i                )                                      ⁡                          (              n              )                                ·                                    c                              (                i                )                                      ⁡                          (              m              )                                                          (        3        )            
of data d(i)(n) transmitted by each node i is, each weighted by coefficient c(i)(m) corresponding to carrier m in the spreading sequence assigned to node i. The result of each demodulation is then multiplied by the corresponding coefficient c(k)(m) of the spreading sequence assigned to node k, the data of which are desired to be received. The products obtained for each carrier m are added. This provides:
                              ∑                      m            =            0                                N            -            1                          ⁢                  (                                                    c                                  (                  k                  )                                            ⁡                              (                m                )                                      ·                                          ∑                                  i                  =                  0                                TN                            ⁢                                                                    d                                          (                      i                      )                                                        ⁡                                      (                    n                    )                                                  ·                                                      c                                          (                      i                      )                                                        ⁡                                      (                    m                    )                                                                                )                                    (        4        )            
Datum d(i)(n) transmitted by each node i is the factor of a term:
                                          ∑                          m              =              0                                      N              -              1                                ⁢                                                    c                                  (                  k                  )                                            ⁡                              (                m                )                                      ·                                          c                                  (                  i                  )                                            ⁡                              (                m                )                                                    ,                            (        5        )            
Due to the orthogonality of the spreading sequences, only datum d(k)(n) transmitted by node k (i=k) is, in node j, the factor of a non-zero term.
More generally, a group of K data d(i)(nK) to d(i)((n+1)K−1) is spread over the N carriers by a multiplication by a spreading matrix with N lines and K columns. In the above example (FIG. 2), the spreading matrix is a single column (formed by column vector c(k)(0), . . . , c(k)(N−1)), all transposed. The coefficients of the spreading matrix are thus a function of the communication to be established. More specifically, the coefficients of the decoding matrix used in receive mode by a given modem must be a function (for example, equal, in the case of real codes) of those of the spreading matrix used in transmit mode by the node from which data are desired to be received.
In such a multicarrier transmission, the propagation delays of the signals on the network and their reflections especially create an interference between the transmitted symbols S(i)(n, t). Such an interference may disturb or prevent the signal reception by the network nodes.
Standards relative to the networks using the power system as a transmission support conventionally provide a frequency-division multiple access performed by using an orthogonal frequency-division multiplexing (OFDM). For example, article “HomePlug Standard Brings Networking to the Home” by Steve Gardner, Brian Markwalter, and Larry Yonge, published in December 2000 in Communication Systems Design, which is incorporated herein by reference, discusses the application of such a multiplexing to networks using electric power cables as a transmission support. A significant disadvantage of an OFDM multiplexing in such a network, however, is that the turning-on of some devices powered by the network (for example, using an electric motor) may cause significant noise on a specific frequency band. If this specific frequency band encroaches upon a frequency band assigned for a data transmission, this transmission can no longer occur.